1. Field of the Invention
The present invention relates to an AD (analog to digital) converter.
2. Description of the Related Art
In a sensor circuit for processing physical quantity, such as acceleration and magnetism, detected by a sensor, offset adjustment is generally performed in order to more precisely process the physical quantity to be detected (see Japanese Laid-Open Patent Publication No. 2001-91373, for example). There is shown in FIG. 7 an example of a sensor circuit 500 for processing an output from a bridge circuit 600 to be used as an acceleration sensor. A preamplifier 610 amplifies an output from the bridge circuit 600, to be output to a delta-sigma AD converter including a delta-sigma modulation circuit 620 and a digital filter 630. The delta-sigma AD converter converts an output from the preamplifier 610 into a digital value, and an output interface circuit 640 outputs the digital value to a microcomputer (not shown.)
As a first example of an offset adjustment method in the sensor circuit 500, a method is cited where only a polarity of a voltage applied to the bridge circuit 600 is changed and the digital value in one state where the polarity of the voltage is not changed and the other state where the polarity of the voltage is changed are compared in the microcomputer (not shown.) Specifically, first of all, the control circuit 650 controls switches SW100 to SW130, so that a power supply VCC is connected to a node VA to which resistors R100 and R110 are connected, and a ground GND is connected to a node VB to which resistors R120 and R130 are connected, respectively. And then, the output interface 640 outputs to the microcomputer (not shown) the digital value in a state where the power supply VCC is connected to the node VA and the ground GND is connected to the node VB. Next, the control circuit 650 controls the switches SW100 to SW130, so that the ground GND is connected to the node VA and the power supply VCC is connected to the node VB, respectively. And then, the output interface circuit 640 outputs to the microcomputer (not shown) the digital value in a state where the ground GND is connected to the node VA and the power supply VCC is connected to the node VB. Thus, in a case where only the polarity is changed of the voltage applied to the bridge circuit 600, the polarity of the output from the bridge circuit 600 is changed, however, the polarities of offsets in the preamplifier 610 and the delta-sigma modulation circuit 620 are not changed. Therefore, an offset of the sensor circuit 500 can be cancelled by comparing the digital values in the above-mentioned different states in the microcomputer (not shown.)
Furthermore, as a second example of the offset adjustment method, there is cited a method of using a chopper amplifier, etc., in order to reduce an offset of the preamplifier 610, for example (see non-patent document: Eric Nolan, “Demystifying Auto-Zero Amplifiers-Part 1,” Analog Dialogue, Analog Devices, Inc., March, 2000, vol. 34-2, pp. 1-3.)
In the case where the polarity of the voltage applied to the bridge circuit 600 is changed and the digital values in the different states are compared in the microcomputer (not shown) as described in the above first example, there is a problem that processing in the microcomputer (not shown) increases due to the adjustment of the offset of the sensor circuit 500. Moreover, in the case where the offset adjustment is performed only for the preamplifier 610 as described in the above second example, there is a problem that the adjustment has no effect on an offset generated in the delta-sigma modulation circuit 620 which is a circuit including an analog circuit other than the preamplifier 610 including that, for example.